1. Field of the Invention
The present invention relates to optical signal processing, and specifically to an apparatus and method for diagnosing a laser beam. More specifically, the present invention relates to an apparatus and method that continuously monitors the laser beam, measuring the radius of curvature of the wavefront, which indicates the extent to which the beam is focusing or de-focusing.
2. Description of Related Art
Beam diagnosis is an issue for many applications of laser technology. Many lasers output a beam with characteristics that may be considered less than perfect; a typical laser beam is far from a perfectly collimated gaussian beam with infinite temporal coherence. Furthermore, as the laser beam is processed or redirected with conventional optics, or as the beam is passed through any of a variety of optical components, its characteristics are often changed in a way that is difficult to diagnose. The components that affect the beam may include amplifiers, or optical components such as mirrors or lenses. Any slight curvature in these components may affect the radius of curvature of the wavefront; i.e., it may cause the beam to be converging or diverging.
A beam of light may be diagnosed according to the curvature of the beam's wavefront. A beam may be collimated, convergent, or divergent. A divergent beam is spreading out; it is "de-focusing". Like a flashlight beam its cross-section will continue to spread as it travels. This de-focusing effect can be seen by campers who shine their flashlight first on a nearby object such as a tent, and then on a faraway object such as a tree. When the light shines on the nearby tent, it has a high intensity and appears in a defined circle; however, the light on a distant tree is spread out and the light circle is not well defined. A flashlight beam is incoherent, which means the phase in a cross-section is random. Furthermore, the flashlight beam is not monochromatic; it consists of a broad band of wavelengths across the visible spectrum. In comparison, a laser beam is substantially monochromatic; it consists of a very small narrow range of wavelengths. Furthermore, a laser beam has a certain amount of spatial coherence, which refers to the alignment in phase between waves travelling side-by-side, viewed in a cross-section. A coherent laser beam can be conceptualized as a series of aligned wavefronts travelling in the direction of propagation of the beam.
A diverging laser beam may be easily produced with a concave lens. With respect to this lens, the diverging laser beam has wavefronts that appear to be convex. By comparison, a convergent beam has wavefronts that appear to be concave. A convergent beam will focus to a small point having a size that is equal to or greater than the diffraction limited size. An example of convergent beam is easily produced by a common magnifying glass. Many a child has found that a magnifying glass may be used to focus the sun's rays. At the focal point, the concentrated rays have enough intensity to burn paper, ants, or an unsuspecting friend.
If a laser beam is neither converging or diverging, then it is said to be collimated. For many laser applications, a collimated beam is highly desirable. The wavefronts in a collimated beam have no curvature, and are perpendicular to the direction of propagation. The radius of curvature is infinite, and the edges of the beam will remain parallel for a great distance. Beyond this great distance, the beam will show some diffraction related divergence.
As stated earlier, the curvature of the wavefronts may be straightforwardly changed by lenses such as convex lenses and concave lenses. Curvature may also be affected by other components in the beam path, such as mirrors, Q-switches and gain material. Any slight curvature in these components, or irregularity in the index or refraction may affect the wavefront's curvature; i.e., it may cause the beam to be converging or diverging. The effect may be multiplied as the beam goes through a number of components; a slight change in curvature at the beginning of a line of components may be translated to a very large change in curvature at the end of the line.
Additionally, wavefront curvature may be affected by the "self-focusing" effect. Particularly it is known that the intensity of the laser beam and the composition of the optical components may affect the curvature of the wavefront; many materials exhibit "self-focusing". A typical laser beam has an intensity profile that has a maximum intensity in the center. As the beam travels through an optical material such as a lens, the relatively high center intensity affects the index of refraction in the material's center to a greater extent than in the surrounding areas which are exposed to a smaller intensity. As a result of these differences in the index of refraction, the material itself causes an unintended focusing effect similar to a lens. This effect is termed "self-focusing". It is known that glass and other conventional laser materials such as dye solutions and crystals exhibit self-focusing. Self-focusing is a function of intensity, and may become uncontrolled. In general, self-focusing affects the radius of curvature of the wavefront in an unpredictable manner.
Many different applications exist for laser beams. Often, an accurate diagnosis of the radius of curvature can make a substantial difference in the success of the end process. If a laser beam can be characterized by its radius of curvature, then beam processing can be applied to produce a beam of the desired curvature. Thus, accurate diagnosis of the radius of curvature of a laser beam can be a useful tool.
For example, in the technology of isotope separation using lasers, laser beams from a number of lasers and amplifiers are combined and directed through a chamber containing vapor of the isotopes to be separated. A perfectly collimated beam is highly desirable and is very important to the success and efficiency of the LIS process. However, delivering a beam of this quality to the separation chamber is a difficult challenge due to the power of the laser beam and the extent of the optical network needed to direct the laser beam to the chamber. If the laser beam going into the chamber could be diagnosed accurately, the radius of curvature could be modified by conventional optical techniques, thereby to provide a laser beam to the chamber with the desired radius of curvature.
Methods have been developed for analyzing a laser beam to determine its curvature. In one method of diagnosing a laser beam, an interferometer such as a shear plate is positioned in the beam path, at an angle to the incoming beam. A typical shear plate comprises a plate of glass having the two opposed planar surfaces formed slightly non-parallel. A sufficiently coherent beam is partially reflected off the front surface, and partially reflected at a slightly different angle off the back surface of the shear plate. Where the two reflected beams combine, a set of interference fringes are created.
The contrast of the interference pattern is much affected by the amount of spatial coherence of the laser beam; a greater spatial coherence will produce sharper fringes. A lesser spatial coherence may still produce an interference pattern, but with less definition of the fringes. There are many sources of coherent laser radiation; lasers such as HeNe lasers, excimer lasers, or metal vapor lasers can produce laser radiation that is coherent to some degree. A well-controlled laser oscillator design can provide a high degree of spatial coherence, but generally such lasers are relatively expensive. Generally it is more cost effective to produce a beam of high spatial coherence by applying a laser beam with low spatial coherence to a conventional spatial filter. A typical spatial filter may include a focusing lens (microscopic objective), a pinhole, and another lens. The beam with low coherence is focused by the microscope objective on one side of the pinhole, and the other lens is positioned in the beam diverging from the microscope objective in order to collimate the beam.
In the technology of optical processing, coherence is generally divided into two types: spatial coherence and temporal coherence. Spatial coherence refers to the uniformity of a cross-section of the beam. A laser beam with high spatial coherence may exhibit a circular intensity profile that is Gaussian; i.e., the peak intensity is at the center of the circular cross-section, and the intensity drops off uniformly to the sides as a Gaussian curve. In this Gaussian cross-section, the electric fields are substantially in phase, thus producing a highly coherent cross-section. Temporal coherence refers to the variation in the beam over time, and is often measured in terms of length. One concept of temporal coherence of a laser beam is in terms of its monochromaticity; a beam that has zero bandwidth (a single wavelength) is completely monochromatic and will exhibit a very long, effectively infinite temporal coherence length. Such zero bandwidth beams are not the norm; real laser beams have a finite temporal coherence. However, temporal coherence is generally not a substantial limitation to creation of an interference pattern, because most interferometers such as shear plates, require a coherence length much smaller than the coherence length of most conventional lasers.
In observing an interference pattern, the shape of the interference fringes is a direct indication of the extent to which the beam is collimated. Shear plates are commonly used by a scientist to observe an interference pattern while physically adjusting the position of a lens in the beam path to provide a collimated beam. When the beam is substantially collimated, the scientist observes the interference pattern as a series of straight, parallel lines that are perpendicular to the propagation of the two beams combining to form an interference pattern. Divergence or convergence is observed as a rotation of the interference pattern, either clockwise or counterclockwise. A problem with this method is accuracy; the scientist will not be able to pinpoint the exact lens position at which the beam is collimated with high precision. In other words, he cannot detect a small amount of convergence or divergence. Furthermore, if he observes a tilting of the fringes in the interference pattern, he cannot accurately ascertain the radius of curvature.
To obtain a more accurate measurement of the radius of curvature of the beam, the interference pattern may be provided to a computer. In a conventional method of beam diagnosis, the interference pattern is monitored by a video camera. A typical video camera includes a conventional CCD (charge-coupled device), which observes the interference pattern as a matrix of pixels, and outputs an electrical signal indicative of the observed image. The image information is then digitized and provided to a computer which analyzes the beam. For example, the angle and degree of curvature of the interference fringes may be analyzed. One disadvantage of this method is the considerable amount of computer processing time required for an accurate analysis; computer analysis of the interference pattern cannot be considered a real time process. Furthermore, the resolution of the CCD is limited; digitizing introduces errors such an an aliasing error. Thus, computer analysis of the interference pattern is difficult and imprecise, and the resulting diagnosis may not be entirely accurate.